Photorefractive materials

ABSTRACT

The present invention relates to polymeric materials that exhibit an erasable photorefractive effect which can be fabricated into optical devices such as optical wave guides.

FIELD OF THE INVENTION

The present invention relates to polymeric materials that exhibit areversible photorefractive effect for use in phase conjugation, hologramformation, amplification, integrated optics, spatial light modulation,optical image processing and storage, and the like.

BACKGROUND OF THE INVENTION

The photorefractive effect involves light-induced charge redistributionin a nonlinear optical material which produces local changes in theindex of refraction such that dynamic, erasable holograms can be formedwhich diffract light. The photorefractive effect is achieved by exposingthe material to an optical intensity pattern consisting of bright anddark regions, such as formed by interfering two coherent laser writingbeams. Mobile charge generated in the material migrates to theappropriate region to form internal space charge electric fields whichdiffract the light during readout with a reading beam in accordance withthe electro-optic effect

Inorganic crystals exhibiting the photorefractive effect are well knownin the art as described in Guenter and Huignard "PhotorefractiveMaterials and Their Applications", Vol. I and II ("Topics in AppliedPhysics" Vols. 61 and 62) (Springer, Berlin, Heidelberg 1988), thedisclosure of which is incorporated herein by reference. Inorganicphotorefractive crystals have been fabricated into optical articles forthe transmission and control (change phase, intensity, or direction ofpropagation) of electromagnetic radiation.

Solid State Communications Vol. 74, pages 867-870, 1990 disclosesorganic crystals of 2-cyclooctylamino-5-nitropyridinedoped with a minoramount of 7,7,8,8-tetracyanoquinodimethane which apparently show thephotorefractive effect.

However, it is technically difficult to fabricate such crystals intodesired thin layered devices such as optical wave guides. Further, it isdifficult to dope organic crystalline material with dopants to achievedesired property improvements such as increase in the speed and/ormagnitude of the photorefractive effect because dopants are oftenexcluded from the crystals during growth.

Therefore, there still is a need in the art for photorefractivematerials which can be readily fabricated into thin film optical devicesand can be doped with sufficient amounts of suitable dopants to achievedesired property improvements.

It is therfore an object of this invention to provide a processablepolymeric material exhibiting a photorefractive effect.

Other objects and advantages will become apparent from the followingdisclosure.

SUMMARY OF THE INVENTION

The present invention relates to an amorphous or substantially amorphousphotorefractive material comprising a polymer, a non-linear opticalchromophore and a charge transport agent, said material having anerasable diffraction efficiency of greater than 1×10⁻⁸ and aphotoconductivity greater than 1×10⁻¹⁴ inverse-ohm-centimeter per wattper square centimeter. Photorefractivity requires that the material havea second-order non-linear optical response. Suitable second ordernon-linear optical coefficients (r₁₃ or r₃₃ depending on the opticalpolarization) are greater than about 0.01 picometer/volt. The secondorder non-linear optical response may be obtained by establishing polarorder of the non-linear optical chromophores in the material by polingthe material with an external electric field. The second-order nonlinearoptical response may also be obtained from the third-order opticalresponse of an isotropic material using an external electric field.Photorefractivity also requires a charge generator and charge traps.Defects inherent within the material will generally function as chargetraps. In some cases, the non-linear optical chromophores will alsofunction as charge generators.

The present invention also relates to optical articles for the controlof electromagnetic radiation which are made from the photorefractivematerial of the present invention A more thorough disclosure of thepresent invention is presented in the detailed description whichfollows.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an amorphous or substantially amorphouserasable photorefractive material comprising a polymer, a non-linearoptical chromophore ("NLO chromophore") and a charge transport agent,said material having a reversible diffraction efficiency greater than10⁻⁸ and a photoconductivity greater than 10⁻¹⁴ inverse-ohm-centimeterper watt per square centimeter. The photoconductivity measurements arenormalized to the intensity of light used.

One major role of the polymer of the photorefractive material of thepresent invention is to function as a binder. The polymer generally istransparent, film-forming, processable and soluble in organic solvents.Other desired properties will be determined by the work environment ofthe photorefractive material such as thermal stability, good adhesion tosubstrates, mechanically tough (non-brittle) and chemically stable inambient environment. The polymer for use in the material of the presentinvention preferably has a molecular weight of greater than about 500and an optical absorption coefficient at the operating wavelength ofpreferably less than about 10 inverse centimeters.

The nonlinear optical chromophore and the charge transport agent can bedispersed in the polymer binder (as a guest/host material) oralternatively, can be covalently bonded to the polymer in the backboneor as a pendant group. Suitable polymer/copolymers for use in thepresent invention include polyesters, polyamides, polyurethanes andpolymers/copolymers comprising monomers having reactive functionalityselected from epoxy, cyanato or maleimido. Other types of suitablepolymers are polyacrylates, polystyrenes, polycarbonates, poly (vinylcarbazoles), poly(arylamines), poly (phenylene vinylenes), poly(phenylene oxides) and poly (vinyl thiazoles). Other types of suitablepolymers will be known to those skilled in the art such as thosepolymers disclosed in U.S. Pat. Nos. 4,886,339; 4,289,842; 4,590,147;4,497,889; 4,535,052 and U.S. Pat. No. Re. 27,925 the disclosures ofwhich are incorporated herein by reference.

If it is desired that the photorefractive material have a permanentsecond order non-linear optical response, a polymer having a high glasstransition temperature (Tg) will be suitable. The high Tg in a polymeris achieved by crosslinking the polymer or by using a thermoplasticpolymer having a high Tg. Preferably such polymer has the non-linearoptical chromophore covalently bonded to the polymer.

Such crosslinked polymers preferably comprise at least onepolyfunctional monomer having, prior to its polymerization, at least onereactive functionality (preferably two or more reactive functionalities)selected from epoxy, cyanato, and maleimido which are known in the artto form crosslinked polymers having a high T_(g). High T_(g) generallycorrelates directly with the stability of induced polar order of apolymer as evidenced by a smaller decrease in loss of second-ordernonlinear optical response over a longer period of time.

Such crosslinked polymers are conveniently made by placing in anelectric field an oligomer comprising preferably a nonlinear opticalchromophore and having the desired reactive functionalities. Theoligomer is heated or exposed to light in the electric field to causecrosslinking of the oligomer. The polar order of the polymer isestablished in the field and after the polymer has been crosslinked, thepolar order of the polymer is locked into place. Preferably, if thepolymer is heated to effect temperature in the field. This process canalso be performed substituting suitable monomers for the oligomer.

Suitable epoxy monomers for use in the material of the present inventioninclude monomers having two or more epoxy ##STR1## reactivefunctionalities. Such monomers can be polymerized to form a crosslinkedhomopolymer or, alternatively, copolymerized with a variety ofcomonomers known to those skilled in the art such as monomers havingreactive functionalities including amino, hydroxy, cyanato, maleimido,carboxy, or anhydrido substituents. A preferred crosslinked polymer ofthe present invention is comprised of monomers having both epoxy andamino reactive functionalities and an NLO chromophore bonded to themonomer. Another preferred crosslinked polymer of the present inventionis a copolymer comprised of a comonomer having at least two epoxyreactive sites and a comonomer having at least two amino reactive sites,with an NLO chromophore chemically bonded to either or both of thecomonomers. Examples of monomers which can be polymerized to form thecrosslinked polymers of the present invention include: ##STR2##

Suitable cyanato monomers for use in the materials of the presentinvention include monomers having two or more (NCO-) reactivefunctionalities. Such monomers can be polymerized to form a crosslinkedhomopolymer with triazine bridging groups as disclosed in Soc. Adv.Matl. Proc. Eng. Ser., Vol. 20, p. 243, 1975 or, alternatively,copolymerized with other comonomers known to those skilled in the art tohave suitable reactive functionalities, such as epoxy functionalities togive oxazoline polymers, alcohol reactive functionalities to givealkylarylimidocarbonate, and maleimide reaction functionalities to givetriazineimidazole. Examples of monomers which may be polymerized to formthe crosslinked polymers of the present invention include: ##STR3##

Suitable maleimido monomers for use in the materials of the presentinvention include monomers having the following structure: ##STR4##where A can be alkyl or aryl and optionally substituted with a varietyof substitutes known to those skilled in the art which will notinterfere with the polymerization reaction or the photorefractiveeffect. A is also preferably bonded to an NLO chromophore. Thesemonomers can be generally formed by reacting maleic anhydride with anappropriate diamine. The monomers can be polymerized with comonomershaving suitable reactive functionalities known to those skilled in theart as amino, alkenyl, or cyanato. Examples of such monomers which canbe polymerized to form crosslinked polymers of the present inventioninclude: ##STR5##

Such crosslinked polymers for use in the materials of the presentinvention preferably have a T_(g) greater than about 100° C., preferablyfrom about 150° C. to about 400° C. or more, and more preferably about250° C. to about 350° C. The higher T_(g) generally correlates directlywith the stability of the polar order of the polymer. Polymers withhigher T_(g) show smaller decrease in the second-order nonlinear opticalresponse over longer period of time.

The nonlinear optical chromophores for the materials of the presentinvention have unsymmetrical, polarized, conjugated π electrons betweena donor and acceptor group. Examples of NLO chromophores are as follows:##STR6##

Other suitable chromophores for use in the present invention will beknown to those skilled in the art such as those disclosed in "NonlinearOptical Properties of Organic Molecules and Crystals" by Chemla andZyss, Academic Press, 1987, the disclosure of which is incorporatedherein by reference. The material of the present invention comprisesabout 10 weight % to about 90 weight % of the NLO chromophore,preferably greater than about 20 weight %. The NLO chromophore providesthe photorefractive material with the requisite second order non-linearoptical response.

Photorefractivity also requires the generation of a mobile charge. Thegeneration of a mobile charge may be accomplished by light absorption ina suitable chromophore and subsequent separation of an electron-holepair. In some cases, the NLO chromophore can also function to generatethe mobile charge which migrates during the photorefractive effect.However, the photorefractive materials of the present invention mayoptionally be doped with charge generating agents if the NLO chromophoredoes not function as a charge generator or if it is desired, to enhancethe response at a desired wavelength.

The charge generating agent can be dispersed in the polymer binder orbonded to the polymer or the NLO chromophore. Suitable charge generatingagents are bis-azo and tris-azo dyes or pigments, squarylium dyes orpigments as disclosed in U.S. Pat. No. 3,824,099; phthalocyaninepigments as disclosed in U.S. Pat. No. 3,898,084; perylene dyes orpigments as disclosed in U.S. Pat. Nos. 4,431,721 and 4,587,189;perylium salts as disclosed in U. S. Pat. No. 4,108,657 or cyanine ormethine dyes. Other types of charge generating agents will be known tothose skilled in the art.

The charge generating agent may absorb light at longer wavelength thanthe NLO chromophore and it will have an oxidation potential larger thanthe charge transport agent (for hole conduction) or a reductionpotential larger than the charge transport agent (for electronconduction). The material preferably comprises about 0 to about 15weight % of the charge generating agent. The photorefractive material ofthe present invention will have an optical absorption coefficient ofabout 0.1 to about 10, preferably less than about 3 inverse centimeters.

The charge transport agent for the material of the present invention maybe one of two general types: a hole transport agent, or an electrontransport agent. The charge transport agent in combination with thecharge generating agent provides the photorefractive material withrequisite photoconductivity of greater than about 10⁻¹⁴ inverse-ohmcentimeter per watt per square centimeter. Hole transport agents may becharacterized as having excess electron density in the neutral statesuch that oxidation to the cation radical occurs easily. The material ofthe present invention comprises about 10 weight % to about 80 weight %of the hole transport agent, preferably at least about 20 weight % sothat the hole may travel easily by hopping or migrating from molecule tomolecule. Examples of classes of hole transport agents are as follows:

hydrazones such as aldehyde-N-diphenyl hydrazones e.g.p-diethylaminobenzaldehyde diphenylhydrazone

carbazoles such as ##STR7## wherein R is alkyl and R' and R" are alkylor aryl

amino-substituted aryl methanes such as ##STR8## where R is alkyl oraryl

aryl amines ##STR9## wherein R is alkyl

pyrazolines ##STR10## wherein R is alkyl or aryl

oxazoles ##STR11## where R is alkyl or aryl

oxadiazoles ##STR12## where R is alkyl or aryl

amino-substituted stilbenes ##STR13## where R is alkyl or aryl

An electron transport agent may be characterized as having a deficiencyof electron density such that in the reduced state electrons may migratefrom molecule to molecule. The material of the present inventioncomprises about 10 weight % to about 80 weight % of the electrontransport agent, preferably greater than about 20 weight %. Examples ofelectron transport agents are nitro-substituted fluorenones such as: 2,7dinitrofluorenone, 2,4,7 trinitrofluorenones, 2,4,5,7tetranitrofluorenone and 2 carboalkoxy - 4,5,7-trinitro fluorenone.

Other suitable charge transport agents for use in the present inventionwill be known to those skilled in the art such as those disclosed in"Electronic Properties of Polymers", Chapter 6, Mort, and Pfister,(Wiley, 1982), the disclosure of which is incorporated herein byreference. The charge transport agent can be dispersed in the polymerbinder or alternative covalently bonded to the polymer to form aphotoconductive polymer such as ##STR14##

The photorefractive effect depends on the action of an internal electricfield produced by trapped space charge. Generally, defects such asmicrovoids, chain ends and conformational variations in the amorphousmaterial will function as charge traps for migrating charge. Spacecharge trapping can optionally be enhanced if necessary or desired, atthe expense of reduced photoconductivity, by incorporation of chargetrapping agents into the material. For example, when the chargetransport agent is an electron donor (hole transport agent), chargetraps can be provided by incorporating into the material a moleculehaving a lower oxidation potential than the charge transport agent. Forexample, if p-diethylaminobenzaldehyde diphenyl hydrazone (0.58 voltsoxidation potential) is utilized as a charge transfer agent, thematerial could be doped with diethylaminobenzaldehydemethylphenylhydrazones (0.53 volts oxidation potential) or1-phenyl-3-diethylaminostyryl-5-(diethylamino) hydrazone (0.51 voltsoxidation potential).

When the charge transport agent is an electron acceptor, (electrontransport agent) charge traps can be molecules having a lower reductionpotential than that of the charge transport agent. For example, if thecharge transport agent is trinitrofluorenone, tetranitrofluorenone, canbe the charge trap. The concentration of the charge trapping agent inthe material is about 0 to 10 weight %.

The amorphous or substantially amorphous material of the presentinvention is generally non-crystalline such that it will notsubstantially scatter incident light. The material is comprised ofcomponents which may be crystalline prior to forming the material.However, the term "amorphous" as used herein is intended to includematerials which are completely amorphous, materials which have thereindispersed areas of crystallization insufficient to cause substantialscattering of light and also materials which have microcrystals smallerthan the wavelength of incident light (e.g. 350 to 700mm.)

The material of this invention has a second order optical nonlinearityfor the electro-optic effect which can be detected using a Mach-Zehnderinterferometer. The interferometer splits a polarized coherent laserbeam into two beams of equal intensity; one is passed through thematerial with transparent electrodes at non-normal incidence and then ismixed with the other beam. When a voltage is imposed across the film inthe 3-direction (normal to the plane of the material) a phase shiftoccurs which is analyzed for the second order non-linear opticcoefficients r₁₃ and r₃₃ (where the subscript 13 denotes 1-direction foroptical polarization of incident light and the 3-direction for theapplied electric field).

The photoconductivity of the photorefractive materials of this inventionmay be determined by applying a voltage across the material andmeasuring with an ammeter the additional current that results when thematerial is illuminated. For example, with the 3-direction being normalto the plane of the material, the voltage may be applied along the3-direction and current flowing in the 3-direction is measured.

To distinguish the materials of the present invention as beingphotorefractive rather than photochromic, photorefractivity may bedetermined in any one of three tests depending upon the relativeimportance of carrier diffusion and external-field-induced drift in thecharge transport process. A positive result on any one of these threetests determines that the material exhibits the photorefractive effect.In all cases the diffraction must be anisotropic consistent with thesymmetry of the electro-optic coefficients for various polarizations.

(1) Reversible anistropic holographic grating formation with the phaseshift between the light intensity pattern and the index of refractionpattern not equal to zero degrees indicates the photorefractive effectin which diffusion dominates over drift in the charge transport process.For example, two coherent beams are overlapped in the material to form agrating. One beam is then attenuated, and the optical phase shiftbetween the transmitted weak beam and the diffracted beam from theremaining strong beam is measured by standard interferometrictechniques; or

(2) The presence of anistropic asymmetric two-beam coupling indicatesthe photorefractive effect in which diffusion dominates over drift. Theasymmetric two-beam coupling can be observed when two coherent beams areoverlapped in the material and the optical power of in the twotransmitted beams is measured by art known techniques. Asymmetrictwo-beam coupling occurs if the optical power of one of the two beamsdecreases while the optical power in the other beam increases duringgrating formation; or

(3) If drift dominates over diffusion in the charge transport process,then the presence of the photorefractive effect can be determined by theformation of an erasable holographic anisotropic diffraction gratingonly when an external DC electric field is applied to the materialduring grating formation.

The reversibility or erasability of the photorefractive effect of thematerial of the present invention is exhibited when the material isilluminated with two coherent writing light beams in the presence of anelectric field for a period of time equal to the product of ten, thedielectric constant, and the permittivity of free space all divided bythe photoconductivity. After removal of one of the light beams, adiffracted beam will appear traveling in the same direction as the beamthat was removed as a result of photorefractive hologram formation. Thephotorefractive hologram can then be erased by applying only one writingbeam of the same wavelength for a sufficient period of time.

The materials of the present invention can be fabricated into opticalarticles for the transmission and control (change phase, intensity, ordirection of propagation) of electromagnetic radiation by art-knowntechniques. The materials of the present inventions can be formed intothin films by casting or by spin coating. Patterned channel waveguidescan be produced with the thin films using standard techniques oflithography or direct laser writing to make waveguide photorefractivedevices, such as phase conjugators, mirrors, amplifiers, spatial lightmodulators, optical processors, or holographic optical storage devices.The methods for making such optical devices are known to those skilledin the art as disclosed in the Gunters reference set forth above.

The following examples are detailed descriptions of methods ofpreparation of certain materials of the present invention. The detailedpreparations fall within the scope of, and serve to exemplify, the moregenerally described methods of preparation set forth above. The examplesare presented for illustrative purposes only, and are not intended as arestriction on the scope of the invention. All temperatures are indegrees celsius.

SYNTHESIS OF POLYMER EXAMPLE 1

Polymer prepared from epoxy reaction of bisphenol A diglycidylether and4-nitro-l,2-phenylenediamine and subsequently mixed withdiethylamino-benzaldehyde-diphenyl hydrazone(DEH): (Bis-A-NPDA/DEH):

Step 1

In a 100 ml round bottom flask with stirbar and nitrogen inlet wasplaced bisphenol A diglycidylether (12.0 g, 35.3 mmol, 70.6 mmol activesites) and freshly recrystallized 4-nitro-l,2-phenylenediamine (3.0 g,19.5 mmol, 77.85 mmol active sites). The resulting mixture was heatedand stirred in a 150° oil bath for 35 minutes. The resulting lowmolecular weight polymer, obtained as a dark glassy solid, had thefollowing properties: Tg 65° ; number average molecular weight (Mn)2200; weight average molecular weight (Mw) 6900 index of refraction=1.62.

Step 2

200 mg of the prepolymer from Step 1 was added to 8 ml of propyleneglycol monomethyl ether acetate and stirred for 1-2 days. 86.4 mg of thetransport agent DEH was added to the mixture to yield a weight fractionof solids of 30%. The dissolved mixture was filtered with a 0.2 micronfilter to remove insoluble material, and utilized to prepare samples. Ina typical sample preparation, 1.5 ml of the mixture was slowly drippedover a time of 30 minutes onto two transparent conducting ITO glassslides maintained at 95 degree C on a hot plate. After this partialdrying procedure, the two glass slides with spacers were pressedtogether and quenched to room temperature on a metal plate. Theresulting samples ranged in thickness from 175 to 500 microns and showedweak optical density gradients which is believed likely due to partialcrosslinking.

EXAMPLE 2

Polymer prepared from epoxy reaction of bisphenol A diglycidylether and4-nitro-1,2-phenylenediamine and subsequently mixed withdiethylamino-benzaldehydenaphthylpheny hydrazone(NDEH):(Bis-A-NPDA/NDEH):

Preparation same as Example 1 above, except that the hole transportagent used is NDEH.

EXAMPLE 3

Polymer prepared from epoxy reaction of bisphenol A diglycidylether and4-nitro-1,2-phenylenediamine and subsequently mixed withdiethylamino-benzaldehyde-N-amino carbazole hydrazone(DECH):(Bis-A-NPDA/DECH):

Preparation same as Example 1 above, except that the hole transportagent used is DECH.

EXAMPLE 4

Polymer prepared from N,N-diglycidyl-4-nitroaniline andN-(2-aminophenyl)-4-nitroaniline and containingdiethylamino-benzaldehyde-diphenyl-hydrazone (NA-APNA/DEH):

Step 1

In a 1000 ml round-bottom flask with stirbar, reflux condenser, andnitrogen inlet was placed 4-bromoaniline (86 g, 500 mmol),epichlorohydrin (184 g, 2000 mmol), and water (0.5 ml). The resultingsolution was gradually warmed to 130° in an oil bath and kept at thattemperature for 150 minutes, after which time only a trace of monoadduct was evident by thin-layer chromatography analysis. The reactionmixture was cooled and excess epichlorohydrin was stripped off by rotaryevaporation, toluene (100 ml) was added, and stripped off again. Thentoluene (100 ml) was added to the stirred solution along with some seedcrystals and methylcyclohexane (100 ml) (added dropwise with stirring).After stirring overnight, the suspension was chilled in an ice bath andfiltered, washed with a cold 1:1 mixture of toluene andmethylcyclohexane, and air dried to give the product as a grey powder(101 g, 56%), mp 100°-112° C. This material was of adequate purity forthe subsequent nitration reaction.

Step 2

In a 1000 ml round-bottom flask with stirbar was placedN,N-(2,2'-dihydroxy-3,3'-dichloropropyl)-4-bromoaniline (35.7 g, 100mmol) and glacial acetic acid (263 ml). To this stirred solution wasadded over about two hours 160 ml of an aqueous solution of sodiumnitrite (55.2 g, 800 mmol). After an additional one hour of stirring,thin-layer chromatography indicated the absence of a bromo startingmaterial and the reaction mixture was transferred to a separatory funnelwith ethyl acetate (500 ml) and water (500 ml). The phases wereseparated and the organic phase was washed with water (200 ml, 2X) andthen saturated sodium bicarbonate solution until gas evolution ceased.The organic phase was dried over magnesium sulfate, filtered through apad of silica gel, and concentrated to a red oil which was dissolved intoluene and stripped down again. Then more toluene (200 ml) was addedand the crude mixture stirred at room temperature, and the resultingcrystalline slurry was stirred at room temperature for 24 hours. Thesolid was isolated by suction filtration and washed with a cold mixtureof toluene and methylcyclohexane to give the 16.4 g of crude product.This material was then recrystallized from a minimal amount of hottoluene to give the desired product in adequate purity for subsequentreaction (12.5 g, 38%), mp 109°-114° C.

Step 3

In a 1000 ml round-bottom flask with stirbar, addition funnel, andnitrogen inlet was placed N,N-(2,2'-dihydroxy-3,3'-dichloropropyl)-4-nitroaniline (12.93 g, 40 mmol), toluene (180ml), and then 45% aqueous KOH (52 g) was added dropwise over 30 minutes.The resulting two-phase system was stirred for 24 hours at which timethin-layer chromatography indicated the reaction was complete. Themixture was transferred to a separatory funnel with ethyl acetate (500ml) and water (200 ml). The phases were separated and the organic phasewas washed with water (100 ml, 2X), then dried over magnesium sulfateand filtered through a pad of silica gel. Concentration of the filtrategave a yellow solid which was dried in a vacuum oven to give the crudeproduct (10.0 g, 100%). This material was recrystallized from a minimalamount of hot methanol to give the pure product as yellow needles (8.2g, 82%), mp 96°-97°: nmr CDCl3δ8.09 (d, 2H), 6.75 (d, 2H), 3.88 (dd,2H), 3.50 (dd, 2H), 3.16 (m, 2H), 2.80 (t, 2H), 2.54 (q, 2H).

Step 4

In a 500 ml round-bottom flask with stirbar, condenser, and nitrogeninlet was placed 1,2-phenylenediamine (10.8 g, 100 mmol),N-methylmorpholine (25.5 g, 250 mmol), 4-fluoronitrobenzene (28.2 g, 200mmol), and anhydrous DMSO (125 ml). The solution was brought to a gentleboil (bath temperature 170°-180°). After 14 hours, the1,2-phenylenediamine was consumed and the solution was cooled andbrought up to 500 ml with water. This solution was transferred to aseparatory funnel with ethyl acetate (300 ml) and water (200 ml). Thephases were separated and the organic phase was washed with water (250ml, 4X), brine (250 ml), and then dried over magnesium sulfate andfiltered through a pad of silica gel. Silica gel was added to thefiltrate and concentrated to dryness. This material was placed at thetop of a column and eluted with a gradient of ethyl acetate in hexane.The appropriate fractions were combined, concentrated, and the resultingorange solid was recrystallized from toluene to give 12.22 g (53%) ofdesired product, mp 142°-145°: nmr (CDCl3δ8.06 (d, 2H), 7.08-7.12 (m,2H), 6.70-6.80 (m, 2H), 6.62 (d, 2H), 5.78 (br s, 1H), 3.76 (br s, 2H).

Step 5

In a 25 ml round-bottom flask with stirbar and nitrogen inlet was placedN,N-diglycidyl-4-nitroaniline (1.1 g, 4.4 mmol, 8.8 mmol active sites)and N-(2-aminophenyl)-4nitroaniline (0.67 g, 2.9 mmol, 8.7 mmol activesites). This mixture was stirred and heated at the following schedule:140° for 3 minutes, 120° for 45 minutes, and finally 130° for 20minutes. After cooling, the prepolymer was obtained as a dark glass withthe following properties:

T_(g) 5°; M_(n) 700; M_(w) 1000; UV_(max) 397 nm index of refraction1.72.

Step 6

Using the prepolymer from Step 5, samples were prepared containing 25 wt% DEH using the techniques described under Example 1, Step 2.

MEASUREMENT OF PROPERTIES Photoconductivity

Light-induced increases in the conductivity of the samples were measuredat zero frequency using a voltage source, a picoammeter and a tunablelaser. Conductivity increases of greater than 1×10⁻¹⁴ inverse-ohm-cmwere observed per watt per square centimeter of incident lightintensity. The measurements for several samples of each of Examples 1-4are as follows:

    ______________________________________                                                  Photoconductivity                                                   Example   (10.sup.-12 inverse-ohm-cm/watt/cm.sup.2)                           ______________________________________                                        1          0.06-12.0*                                                         2         0.02-0.7*                                                           3         0.33                                                                4          0.2-2.8*                                                           ______________________________________                                         *several samples measured.                                               

Second-order Nonlinearity

Samples were studied in a Mach-Zehnder interferometer to obtain a valuefor the electro-optic coefficient. Since the samples were not fullycross-linked, samples could be poled in an electric field in a nearlyreversible fashion at room temperature. The value of electro-opticcoefficient rose linearly with applied electric field, reaching a ofmaximum in the range of 0.02 to 4.0 picometers/volt at an applied fieldof 120 kV/cm. The range of measurements for several samples of each ofExamples 1-4 are as follows:

    ______________________________________                                        Example   Electro-optic Coefficient [.sup.pm /V]                              ______________________________________                                        1         0.02-1.2*                                                           2          0.05-0.24*                                                         3         0.035-0.28*                                                         4         0.33-4.0*                                                           ______________________________________                                         *several samples measured.                                               

Photorefractive Diffraction Efficiency

Using a four-wave mixing geometry known in the art, two mutuallycoherent interfering writing beams overlapping at angles between 1 and85° (typically 30°) at a wavelength of 647 nm were used to write a phasehologram in the material. A third reading beam was used to produce adiffracted beam from the grating. A reading beam was sent through thematerial in a direction opposite (counterpropagating) one of the writingbeams, designated beam B. The resulting diffracted beam appeared in adirection opposite (counterpropagating) to that of writing beam C andmay be measured as a reflection from a beam splitter placed in beam C.Diffraction efficiencies are measured by recording the ratio between thepower in the diffracted beam and the power in the transmitted readingbeam. The range of measurements for several samples of each of examples1-4 are as follows:

    ______________________________________                                        Example     Diffraction Efficiency                                            ______________________________________                                        1             5 × 10.sup.-7 -5 × 10.sup.-5                                                   (at 125 kv/cm)*                                    2             4 × 10.sup.-6 -10 × 10.sup.-6                                                  (at 110 kv/cm)*                                    3             9 × 10.sup.-6                                                                        (at 110 kv/cm)                                     4           1.2 × 10.sup.-5 -1 × 10.sup.-3                                                    (at 85 kv/cm)*                                    ______________________________________                                         *several samples measured.                                               

The sample could only be read out with an external electric fieldapplied to establish the nonlinearity. The gratings showed anisotropicdiffraction as required by the large ratio between the electro-opticcoefficients r₃₃ /r₃₁. Each sample exhibited photo refractivity asdefined by test #3 as previously described. The gratings could be erasedwith either of the writing beams alone, optionally with an electricfield applied of substantially the same magnitude as required forgrating formation. After it has been erased, the material is prepared toaccept the formation of a new grating pattern.

Although this invention has been described with respect to specificembodiments, the details thereof are not to be construed as limitationsfor it will be apparent that various embodiments, changes, andmodifications may be resorted to without departing from the spirit andscope thereof, and it is understood that such equivalent embodiments areintended to be included within the scope of this invention.

We claim:
 1. An amorphous photorefractive material comprising a polymer,a non-linear optical chromophore and a charge transport agent, saidmaterial having a diffraction efficiency greater than 10⁻⁸ and aphotoconductivity greater than 10⁻¹⁴ inverse-ohm-centimeter per watt persquare centimeter.
 2. The material of claim 1 wherein said materialcomprises at least about 20% by weight of said charge transport agent.3. The material of claim 2 wherein said polymer is polyester, polyamide,polyurethane or a polymer comprising one or more epoxy, cyanato andmaleimido monomers.
 4. The material of claim 3 wherein said polymer iscrosslinked.
 5. The material of claim 3 wherein said charge transportagent is an aryl amine, carbazole or a hydrazone.
 6. The material ofclaim 3 wherein said non-linear optical chromophore is covalently bondedto said polymer.
 7. The material of claim 2 wherein said materialfurther comprises a charge generating agent.
 8. The material of claim 2wherein said material further comprises a charge trapping agent.
 9. Anoptical article for the transmission of electromagnetic radiationcomprising an amorphous photorefractive material comprising a polymer, anon-linear optical chromophore and a charge transport agent, saidmaterial having a defraction efficiency greater than 10⁻⁸ and aphotoconductivity greater than 10⁻¹⁴ inverse-ohm-centimeter per watt persquare centimeter.
 10. The article of claim 9 wherein the materialcomprises at about least 20% by weight of said charge transport agent.11. The article of claim 10 wherein said polymer is polyester,polyamide, polyurethane or a polymer comprising one or more epoxy,cyanato and maleimido monomers.
 12. The article of claim 11 wherein saidpolymer is crosslinked.
 13. The article of claim 11 wherein said chargetransport agent is an aryl amine, a carbazole or, a hydrazone.
 14. Thearticle of claim 11 wherein said non-linear optical chromophore iscovalently bonded to said polymer.
 15. The article of claim 10 whereinsaid material further comprises a charge generating agent.
 16. Thearticle of claim 10 wherein said material further comprises a chargetrapping agent.
 17. The article of claim 10 wherein said article is anoptical wave guide.